Forkball v. Splitter is a difference in control for Sasaki:
That makes the forkball an extremely nasty pitch, but it's a double-edged sword, because it's been hard for Sasaki to command. He's had a hard time locating it where he wants to -- either dropping it down below the zone as a chase pitch, or just throwing it for strikes. And Sasaki's erratic command has been his No. 1 issue in the Majors.
The split also has about twice as much spin as the fork. The splitter spins at over 1,000 rpm, compared to the 500-ish rpm on the forkball. That makes it move a lot more consistently -- it fades down and away out of Sasaki's hand -- and a lot less knuckleball-y.
His new, harder splitter is more of an all-purpose pitch. Sasaki can command it better than the forkball. He can either use it to get a strike (his splitter is in the strike zone 44% of the time, compared to 33% for the forkball) or put a hitter away (he's generating a 43% swing-and-miss rate with the splitter and has nine strikeouts with it in three starts, already as many as he has with the forkball all season).